The present invention relates generally to solid state switches, and more particularly to a multiphase solid-state contactor including a power device such as a thyristor or a triac (bi-directional triode thyristor). A solid-state switch has no part that experiences electrical or mechanical wear as in the case of a mechanical switch, and is therefore capable of withstanding frequent opening and closing cycles over a prolonged service period. Because of this advantage, the solid-state switch is extensively used for opening and closing the circuits of induction motors and many other loads.
A method is known for starting and stopping a three-phase induction motor by the use of a solid-state relay having the configuration shown in FIG. 6 (this type of relay is hereinafter abbreviated as SSR). As shown in FIG. 6, the SSR (generally indicated at 1) has as its principal contact a triac 4 including two output terminals 2 and 3. The triac 4 is controlled by an electronic control circuit 5 connected to its gate. The electronic control 5 includes a firing circuit 6 and a photocoupler 8, and the firing circuit 6 includes a zero-crossing circuit 7 which allows the triac 4 to conduct in a substantially zero phase. The firing circuit 6 is connected to the phototransistor in the photocoupler 8, and the light-emitting diode in the photocoupler 8 is connected to two input terminals 10 and 11 via an input circuit 9.
The SSR may be composed of two units, SSRa and SSRb, as shown in FIG. 7. In this case the output terminal 2 of SSRa is connected to the R phase of a three-phase power source RST, while the output terminal 2 of SSRb is connected to the T phase. The ouput terminal 3 of each SSR unit and the S phase are connected to a three-phase induction motor 12 to form the principal circuit. The two input terminals 10 and 11 of each SSR unit are arranged in parallel and connected to a DC power source 14 via a switch 13. When switch 13 is closed, the motor 12 starts by conducting SSRa and SSRb. When switch 13 is opened, the motor 12 stops, since SSRa and SSRb are brought into a non-conducting state. If desired, three SSR units may be used, with each unit being inserted between each phase of the three-phase AC power source and the induction motor 12.
A solid-state relay having the configuration shown in FIGS. 6 and 7 requires that individual SSR units be used in aggregation. Connection of the principal circuit to the control circuit in such a configuration is extremely complex, and involves many separate steps or operations in the manufacturing process. In addition, at least four SSR units are required for the motor to be rotated in both forward and reverse directions, and the wiring of these four SSR units is complicated and time consuming. Compared with mechanical contactors, the SSR usually requires a DC power source for control purposes. In these cases, because of the absence of any difference between the operating voltage (closed-circuit voltage) and the return voltage (open-circuit voltage), unnecessary opening and closing operations occur in response to a variation in the control power source. The SSR, which does not have any auxiliary contacts, has the additional disadvantage that it has no self-holding capability and is unable to produce an auxiliary output that is actuated simultaneously with the opening or closing of the principal circuit.
One object, therefore, of the present invention is to provide a multi-phase solid-state contactor that is free from any of the defects of the prior art system, and which permits easy wiring operations and which can be handled with great ease.
Another object of the present invention is to provide a multiphase solid-state contactor that has the same construction and function as that of a well-known electromagnetic contactor, and which, hence, can be handled as easily as said electromagnetic contactor.
Additional objects and advantages will be obvious from the description of the invention, or may be learned by practice of the invention.